"KY: There’s no real standard for it per se. We needed to made Gr.3 cars for each of those manufacturers for the FIA championship. To make those cars is quite a bit of effort. They’re actually very hard to make properly; to make an existing GT3 car is not as hard, but in order to make a fantasy GT3 car it takes a lot of effort in the design stages.
GTP: Especially cars like the RCZ – front-engined, front-wheel drive becomes mid-engined, rear-wheel drive.
KY: To think about those kinds of designs and work them out is really fun. But at the same time a car that would normally take six months to produce takes a year to produce. That really ramps up the amount of work that our teams have to do."
Already had this assumption but at least Kaz confirmed it, This explains the bare bones car list at launch that was more focused on race cars, the focus on designing and creating those fictional Gr4/Gr3 cars to meet the FIA competition aspect of the game and the lack of previous road cars at launch which we've finally started to see in the free DLC.
Moving forward I'd rather they just recreate more real GT3 cars instead of designing fictional ones to be honest.
It takes way too much time to create those fictional GTs, I'd rather they put in more real cars that take half the development time.